1. Technical Field
The present disclosure relates to porous silica particles, and specifically to porous silica microspheres having modified surfaces that can be useful as functional supports in a variety of applications.
2. Technical Background
Functional supports are useful in many applications, including chromatography, solid phase catalysts, solid phase synthesis of polypeptides and oligonucleotides, and sequencing of polypeptides. Porous silica particles are widely used support materials as the stationary phases for reversed-phase liquid chromatography applications. In such applications, excellent mechanical stability, high surface area, and easily tailored pore size distributions make silica a superior material relative to other supports in terms of efficiency, rigidity, and performance. The surface of silica particles can also be modified to allow a manufacturer to custom produce a variety of stationary phases having tailored chromatographic selectivities.
Silanes are commonly used surface modifying reagents in liquid chromatography applications. For example, “An Introduction to Modern Liquid Chromatography,” Chapter 7, John Wiley & Sons, New York, N.Y. 1979; J. Chromatogr. 352, 199 (1986); J. Chromatogr., 267, 39 (1983); and Advances in Colloid and Interface Science, 6, 95 (1976) each disclose various silicon-containing surface modifying reagents. For reversed-phase chromatography, a traditional silane coupling agent is Si(CH3)2(C18H37), where C18H37 (octadecyl group) yields a hydrophobic surface. In order to covalently attach these silyl groups, silanol (Si—OH) groups are needed on the silica surface.
Silica particles can be strengthened by heating at about 900° C., but such treatment can result in a dehydroxylated silica surface comprising siloxane groups (Si—O—Si), which are generally not reactive with silanizing agents. Accordingly, a rehydroxylation step is typically performed to regenerate silanol groups on the surface prior to silanizing the silica particles.
Common rehydroxylation methods include boiling calcined silica particles in water for extended periods of time or in dilute nitric acid or hydrochloric acid from several hours to up to several days. For example, U.S. Pat. No. 5,108,595 to Kirkland et al. discloses a rehydroxylation process to prepare silanol-enriched surfaces in water in the presence of a basic activator such as ammonium hydroxide, or in the presence of HF solution. While such methods can rehydroxylate the silica surface, the pore structure of the silica particles can be adversely affected. For example, as disclosed in U.S. Pat. No. 5,108,595, equilibrium is reached during the boiling process in the presence of the basic activator or HF solution and silica repeatedly dissolves and precipitates back to the surface of the particles, effectively eliminating micropores and widening mesopores in the silica surface. Also, during cooling, the silica dissolved in the solution can precipitate on the particle surface, forming a new surface layer. As a result, the silica particles after rehydroxylation can have a lower surface area and larger pores than desired.
Recently, hybrid silica particles have become popular for HPLC columns as a new generation HPLC column packing material because of their stability in high pH environments and their physical strength under higher pressures. U.S. Pat. No. 6,686,035 to Waters et al. discloses a method to prepare a porous inorganic/organic hybrid material for chromatography applications. The hybrid silica material is modified by hydrothermal treatment by preparing a slurry in an autoclave at an elevated temperature, e.g., 143° C. to 168° C. and a pH of from 8.0 to 10.7 for a period of 6 to 28 hours. The hydrothermal treatment enlarges the openings of the pores as well as the pore diameters, and forms silanol groups on the surface. The resulting hybrid silica surface has less silanol groups because of the presence of surface organic groups (e.g., methyl groups) which lead to lower bonded phase surface concentrations after silanization, presumably due to the unreactive organic groups on the surface. U.S. Pat. No. 6,528,167 discloses a method to replace surface organic groups on such hybrid materials with surface silanol groups by reacting the hybrid materials with aqueous H2O2, KF, and KHCO3 in an organic solution. U.S. patent Publication No. 2006/0070937A1 to Rustamov et al. discloses a method to form a multilayer article of inorganic/organic layers by surface chemical polycondensation of trifunctional and difunctional organosilanes. This process, however, involves multiple repeating steps of reacting trifunctional and difunctional organosilanes, hydrolyzing any unreacted leaving groups, and dehydrating the sorbent.
Thus, there is a need to address the aforementioned problems and other shortcomings associated with traditional porous silica particles. These needs and other needs are satisfied by the compositions and methods of the present disclosure.